Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Similarly, other thermal or pyrolysis reactors, including reverse flow and other regenerative reactors, are also known for cracking hydrocarbons and/or executing thermal conversions and chemistry processes, including some processes that may be performed at temperatures higher than can suitably be performed in conventional steam crackers. Higher temperature reactions and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions, with equipment temperature, strength, and toughness limitations commonly defining upper limits for many of the processes and facilities.
In an exemplary thermal processing example, the known art discloses that to efficiently obtain relatively high yields of acetylene from thermal processing of methane feed, such as in excess of 75 wt %, reactor temperatures are required to be in excess of 1500° C. and preferably in excess of 1600° C., with relatively short contact times (generally <0.1 seconds). It is known that acetylene may be thermally manufactured from methane in relatively small amounts or batches, using high temperature and short contact time in cyclical processes, yielding a mixture of acetylene, CO, and H2. Methane cracking processes, however, have been inefficient compared to other commercial processes for generation of acetylene and as compared to other cracking processes such as conventional steam cracking to produce olefins. Commercially, acetylene is typically generated by cracking feeds other than methane. The high temperature processes (e.g., >1500° C.) have traditionally not scaled well and are generally only useful for relatively high-cost, specialty applications. Processes such as thermally cracking methane to acetylene have largely been commercially unattractive due to thermal, chemical, and mechanical degradation of the reactor and related equipment. In addition to physical temperature limitations for reactor materials, many prior art reactor materials that are inert at lower temperatures may become susceptible to chemistry alterations at high temperature, leading to premature equipment degradation and/or process interference, such as by generation of contaminants. Although regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved commercial or widespread use, due at least in part to the fact that they have not been successfully scaled well to a commercially economical size or commercially useful life span. These drawbacks have resulted in use of compromised or alternative solutions, such as in the above example, commercial acetylene production is primarily accomplished via processing higher weight hydrocarbons such as ethane, propane, naphthas, and gas-oils at lower temperatures, such as by conventional steam crackers.
Further complicating the material stability and reliability issue has been exposure to large, cyclic temperature swings encountered during many pyrolysis processes. Such temperature changes and product flow direction changes can impose severe physical strength and toughness demands upon the refractory materials at high temperature. Material life expectancy at high temperature can be severely limited or precluded. Such physical demands have also typically limited manufacturing and use of refractory materials to relatively simple shapes and components, such as bricks, tiles, spheres, and similar simple monoliths. Reactor component functions and shapes have been limited for high severity services. For example, a deferred combustion, regenerative reactor process was proposed in a U.S. patent application filed Dec. 21, 2006, Ser. No. 11/643,541, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems. Although the disclosed process of the '541 application effectively controls the location of combustion within the reactor, the internal reactor components must still contend with the severely high temperatures, temperature changes, and physical stresses incurred during methane pyrolysis, particularly for a commercially desirable reactor life term. The refractory material comprising the reactive regions may typically be a ceramic or related refractory material. In some embodiments, however, the disclosed processes and apparatus may utilize relatively complex shaped refractory components, such as a thin-walled honeycomb monolith used to conduct process fluids through the reactor. Such reactors and reactor component geometries may demand materials that have strength, toughness, chemical inertness, and other required properties that exceed the capabilities of previously identified or known refractory materials under such temperature and stress conditions.
For further example, the “Wulff” process represents one of the more preferred commercial processes for generation of acetylene. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, such materials have demonstrated insufficient strength, toughness, and/or chemical inertness, and are not amenable to use as certain desirable reactor components, such as for use as reactor fluid conduits, to facilitate large-scale commercialization. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes (e.g., >1500° C., >1600° C., and even >1700° C.) has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors.
Due to high temperatures involved in cyclic pyrolysis reactors, generally only ceramic components have the potential to meet the materials characteristics needed in such aggressive applications. The American Society for Testing and Materials (ASTM) defines a ceramic article as “an article having a glazed or unglazed body of crystalline or partly crystalline structure, or of glass, which body is produced from essentially inorganic, non-metallic substances and either is formed from a molten mass which solidifies on cooling, or is formed and simultaneously or subsequently matured by the action of the heat.” Ceramics components generally can be categorized in three material categories: engineering grade, insulation grade, and refractory grade.
The term “engineering grade” has been applied to ceramic materials which typically have very low porosity, high density, relatively high thermal conductivity, and comprise a complete component or a lining. Examples include dense forms of aluminum oxide (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), silicon aluminum oxynitride (SIALON), zirconium oxide (ZrO2), transformation-toughened zirconia (TTZ), transformation-toughened alumina (TTA), and aluminum nitride (AlN). These materials usually possess high strength and toughness, which have been dramatically improved to the degree that ceramics are now available that can compete with metals in applications previously thought impossible for ceramics. Strength is a measurement of the resistance to formation of a crack or structural damage in the material when a load is applied. Toughness is a measurement of the resistance of the material to propagation of a crack or extension of damage to the point of failure. For instance, engineering grade Al2O3 and SiC are commercially available with a strength of over 345 MPa, and Si3N4 and TTZ are available with strengths above 690 MPa (100 kpsi). Some TTZ materials have toughness around 15 MPa·m1/2, which is an order of magnitude higher than that of conventional ceramics. Even though engineering grade ceramics have superior strength and toughness at relatively low temperatures, they are relatively poor in thermal shock resistance (both strength and toughness) and many grades, such as but not limited to borides, carbides, and nitrides are not chemically stable at high temperature. Many are also not suitable for use at the high temperatures encountered with some pyrolysis reactions.
The second category of ceramic materials is insulation grade ceramics, which are typified by relatively high porosity. Many may have fibrous crystalline grain structures and are more porous than engineering grade ceramics, have lower density, and have lower thermal conductivity than engineering grade ceramics. Insulating monolithic ceramics and composite ceramics are often fabricated into various forms such as rigid boards, cylinders, papers, felts, textiles, blankets, and moldables. Many are primarily used for thermal insulation at elevated temperatures, such as up to 1700° C. A broad range of porosities and pore sizes can be produced, depending on the intended application, but in general, insulation grade ceramics tend to be relatively porous as compared to engineering grade ceramics. Porous ceramics have many open or closed internal pores that provide the thermal barrier properties. Often, quite porous ceramics, such as those having porosity of greater than 50 vol. % and commonly even in excess of 90 vol. %, are used for thermal insulation where extremely low thermal conductivity (<0.08 W/m·K) is required. However, insulation grade ceramics typically lack the structural strength and functional toughness needed for the internal components of many pyrolysis reactors and processes. Insulation grade ceramics typically are recognized as having a flexural strength or toughness of less than about 4 Kpsi (27.6 MPa) and often of less than even 1 Kpsi (6.9 MPa). Also, the insulation properties of porous ceramics may tend to degrade as the pores may fill with coke accumulation.
The third generally recognized category of ceramic materials is refractory grade ceramics. Many refractory grade ceramics typically have porosity, strength, and toughness properties intermediate to such properties in engineering grade and insulation grade. Refractory grade ceramics typically have thermal shock resistance properties similar to some insulation grade ceramics but higher than engineering grade ceramics. Conversely, refractory grade ceramics typically lack the strength and toughness of engineering grades ceramics, but which properties exceed those of insulation grade ceramics. However, typically as strength increases, thermal shock resistance and related properties are compromised.
All relevant properties must be considered when selecting a ceramic for a particular application. Other relevant ceramic properties or characteristics include but are not limited to maximum use temperature, thermal conductivity, modulus of rupture, modulus of elasticity, electrical resistance, average grain size, density, porosity, and purity. The maximum use temperature is the highest temperature to which refractory ceramics can be exposed without degradation. Thermal conductivity is the linear heat transfer per unit area for a given applied temperature gradient. The modulus of rupture (MOR) or cross-break strength is the maximum flexural strength that refractory ceramics can withstand before failure or fracture occurs. Young's modulus or the modulus of elasticity is a material constant that indicates the variation of strain produced under an applied tensile load. Average grain size measures the size of individual grains or crystals within the microstructure of a polycrystalline ceramic material. Density is the mass per unit of bulk volume. Purity is the percentage, by weight, of major constituents.
As compared to insulation grade ceramics, refractory grade ceramics tend to be stronger across broader temperature ranges. Refractory grade ceramics also generally tend to be more resistant to thermal shock than engineering grade ceramics. However, while some refractory grade ceramics tend to be somewhat inert or chemically stable at elevated temperatures, some refractory grade ceramics become chemically and/or structurally unstable at elevated temperatures, rendering them unsuitable for applications exposed to chemical reactions. Exemplary chemically and/or thermally unstable ceramics include certain silicas, aluminas, borides, carbides, and nitrides. Also, some refractory grade ceramics are known to possess lower thermal conductivities and coefficients of expansion than certain other refractory or engineering grade ceramics. Refractory grade ceramics are also known to undergo alterations in crystalline structure at elevated temperatures. Such alterations can result in changes in bulk volume which can result in production of stress fractures and/or cleavage planes which can reduce the material's strength. Some exemplary, common high temperature refractory grade materials include but are not limited to magnesia (MgO), lime (CaO), and zirconia (ZrO2).
Some engineering grade alumina or zirconia ceramics may provide superior flexural strength, but their thermal shock resistance is poor. Some advanced engineering ceramics, such as SiC and Si3N4, also provide superior strength, but their thermal shock resistance in grossly inadequate. Moreover, these silicon based ceramics can not be used at high temperatures (i.e. >1500° C.) due to high temperature oxidation issue. On the other end of the spectrum lie the insulation grade ceramics. These ceramics offer excellent thermal shock resistance, but they fall quite short of the required strength performance.
The reviewed art is void of teaching how to prepare or select a material having a range of properties that are suitable for use in constructing a furnace for performing substantially continuous, cyclical, high temperature pyrolysis chemistry. Also, materials testing methods commonly applied to metals and polymers are frequently less useful for testing ceramics. The available tests provide only a limited picture of the total performance limits of any particular ceramic. Further complicating the ceramic material selection process is the complicating fact that, like metals and polymers, the performance of a ceramic is also a function of temperature, with temperature-dependent changes in properties such as brittleness, elastic, plastic and viscoplastic deformation, hardness, fatigue, corrosion resistance, and creep resistance. Other important performance factors include but are not limited to thermal shock resistance, thermal expansion, elastic modulus, thermal conductivity, strength, and fracture toughness.
The identified prior art pertaining to refractory materials for high-severity hydrocarbon pyrolysis dates primarily to the 1960's and earlier. However, that art merely occasionally provides generalized lists of some exemplary materials such as ceramics, alumina, silicon carbide, and zircon as reactor materials. These sparse, non-specific disclosures left the art largely incapable of providing a large-scale, commercially useful reactor or reactor process. The teachings of the art was only effective for enabling relatively small scale specialty applications that see vastly inferior use as compared to large scale processes such as hydrocarbon steam cracking. The identified art is void of teaching or providing a refractory ceramic material that has the complex set of properties that are required for extended use in the reactive or other most-demanding regions of a high-severity (≧1500° C.) pyrolysis reactor for the commercial production of acetylene and/or olefins. The studied art does not teach preferred crystalline structure or composition for particular reactor furnace uses, or for complex reactor component shapes and/or functions. Multi-modal ceramics are also known in the art, as are ceramic compositions utilizing nanoparticles. However, specific formulations or teachings for refractory materials having particular utility in high temperature (>1500° C.), high stress, chemically active, thermal reactor applications have not been identified or located in the known art. The studied art is believed to be similarly deficient at teaching materials suitable for complex, irregular, or functionally-shaped reactor components. The art needs a material (e.g., ceramic) that can endure prolonged exposure to high severity temperatures, substantial temperature swing cycles, cyclic flows of combustion and reaction materials, and concurrently provide the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness in the presence of high temperature chemical reactions that is required for large scale, high productivity applications. Lack of materials availability and selection criteria for identifying the materials for use in the reactive and most severe temperature regions of a reactor system is one of the most critical remaining issues in design and large-scale commercial operation of such reactors and processes.